The DMSO currently available on the market is a product which is already of good purity. Its commercial specifications are generally:
purity: .gtoreq.99.7% by chromatography
acidity:.ltoreq.0.04 mg KOH/g by potentiometry
crystallization
point: .gtoreq.18.2.degree. C.
visual
appearance: clear
water content: .ltoreq.0.15% by weight relative to the total weight
colour (APHA): .ltoreq.10
Patent application FR 2,014,385 describes a process for the preparation of purified DMSO. This process uses a cationic ion exchanger which can be regenerated by a treatment with acids. On the other hand, the DMSO treated with this ion exchanger has a very high water content. Furthermore, the examples indicate the use of a strongly basic resin of the Amberlite IR-A 400 or Merck III type for dimethyl sulphide/DMSO/10% sulphuric acid ternary mixtures, which possibly leads to the neutralization of the acidity but also the introduction of the counterion of the basic resin into the mixture thus treated. Indeed, in this process the purification appears to be afforded essentially by fractional distillation of an aqueous DMSO solution treated beforehand with one or more ion exchangers.
Analyses of traces of metals have now been carried out on several samples (1 to 6) of commercial DMSO, from different sources. These analyses are reported in Table I.
The sodium, iron, potassium, calcium, chromium, copper, nickel and zinc concentrations were measured by ICP (plasma-torch atomic emission spectrometry, Perkin Elmer machine, Optima 3000 model) and are expressed in ppb (1 ppb=1 part by weight per billion=1 .mu.g per kg).
The list of metal elements featured in Table I is not exhaustive as regards the metal elements present in these samples.
TABLE 1 ______________________________________ Metal cations Sample Na Fe K Ca Cr Cu Ni Zn ______________________________________ 1 40 13 60 20 2 10 8 10 2 39 60 3 13 13 &lt;2 18 3 3 30 40 3 20 12 &lt;2 15 3 4 30 40 3 14 13 &lt;2 15 3 5 30 &lt;1 20 25 &lt;2 &lt;2 &lt;3 &lt;3 6 70 90 65 55 15 2 25 60 Detection 2 1 3 2 2 2 3 3 limit ______________________________________
For certain applications such as, for example, in electronics or in pharmacy, the DMSOs analysed above contain too many metal impurities. In general, a DMSO containing less than 10 ppb of each alkali metal and alkaline-earth metal contaminant would be necessary for most of the uses in the two abovementioned technical fields.